Copper alloy rocket engine combustion chamber linings have been found to deteriorate when exposed to cyclic reducing/oxidizing (redox) environments which are a consequence of the combustion process. This deterioration, known as blanching, can be characterized by increased roughness and bum through sites in the wall of the combustion chamber lining and can seriously reduce the operational lifetime of the combustion chamber.
One illustrative example of a copper alloy rocket engine combustion chamber that undergoes blanching is in the space shuttle main engine (SSME) propulsion system. A high pressure, high temperature rocket engine, the SSME burns a mixture of liquid oxygen and liquid hydrogen. During combustion, localized regions along the combustion chamber's wall lining become, alternatively, rich in oxygen (forming an oxidizing environment) and rich in hydrogen (forming a reducing environment). When a region of the combustion chamber's lining is exposed to an oxidizing environment, copper oxides form. Later, when exposed to a reducing environment, these copper oxides are reduced. The result of cycling a region of the chamber wall between an oxidizing and reducing environment is to cause the wall lining to become scarred and rough. This, in turn, can result in localized hot spots that reduce the operational (e.g., useful) lifetime of the combustion chamber.
One means of combating blanching is to coat the combustion chamber of a rocket engine with a protective lining as shown in FIG. 1. The published literature documents the long felt need for a rocket engine combustion chamber coating to reduce the blanching problem. See, for example, [D. Morgan, J. Franklin, A. Kobayashi, and T. Nguyentat, "Investigation of Copper Alloy Combustion Chamber Degradation by Blanching," Advanced Earth-to-Orbit Propulsion Technology Conference, May, 1988]. Recognized requirements for such a protective coating include: 1) oxidation and blanching resistance to 1200.degree. to 1400.degree. F., 2) minimum operational lifetime of 100 combustion cycles, 3) diffusional stability with respect to the copper alloy substrate, 4) good thermophysical properties--for example, high through-thickness thermal conductivity and thermal expansion compatible with copper alloy substrates, 5) insensitive to hydrogen diffusion, 6) minimal adverse effect on mechanical properties of the copper alloy substrate--for example, tensile strength, ductility, and low-cycle fatigue life, and 7) resist creep, thermal shock, and thermal fatigue.